The present invention provides a method for controlling the concentration profile of dopants incorporated into a glass article. More particularly, the present invention relates to an improved method of doping an optical fiber preform with dopants that are not readily incorporated during the initial fabrication process.
Optical fibers are typically drawn from glass preforms. An optical fiber preform is generally comprised of a central core and an outer cladding layer. The central core, for the most part, has a higher refractive index than the cladding layer. When the preform is drawn into an optical fiber, the difference in refractive indices between the core and cladding layers allows the propagation of the optical signal within the core. Optical fiber preforms and waveguides are composed primarily of high purity silica glass.
Variations in the refractive index are obtained by adding dopants to layers within the central core or surrounding cladding layer. Certain dopants such as the oxides of titanium, germanium, aluminum, and phosphorous are added, in a weight percentage ranging from about 0.1 to 25%, to increase the refractive index of the glass. Other dopants such as fluorine and boron oxide may be added in similar amounts to decrease the refractive index of the glass. A typical optical fiber glass core composition is comprised mainly of high purity SiO2 glass, in a weight percentage above 50%, with lesser amounts of GeO2, TiO2, and/or other dopants, depending upon the desired optical properties.
Glass optical fiber preform can be made from a variety of processes. Typical processes for making these preforms are variations of chemical vapor deposition (CVD) processes such as Outside Vapor Deposition (OVD), Modified Chemical Vapor Deposition (MCVD), or Vapor Axial Deposition (VAD). These processes typically involve the oxidation of glass precursors, such as metal chlorides, to form glass particulate. Glass precursors, such as silicon tetrachloride (SiCl4), germanium tetrachloride (GeCl4), or titanium tetrachloride (TiCl4) which may be liquid at room temperature, are heated within bubblers, vaporizers, or similar means to form a metal chloride vapor. Chlorides are widely used because they vaporize at relatively low temperatures prior to transportation to the reaction zone or a hot zone such as a burner flame, plasma, or heated area within a substrate tube. The chloride vapors oxidize within the reaction zone thereby forming a glass particulate. After a sufficient thickness of particulate is reached, the glass particulate eventually forms the porous soot blank. The porous soot blank is then sintered, or heated until the pores are eliminated, to form a glass preform. In the MCVD process, the formation and sintering of the glass particulate generally occurs simultaneously.
There is also a need to produce optical fibers with dopants such as rare earth and alumina, most of these dopants which are solids at room temperature sublime, rather than boil, to form a vapor. The vapor pressures of these dopants are low at the temperature ranges commonly used for conventional vapor delivery processes, i.e., about 200xc2x0 C. or below. These properties make it difficult to deliver rare earth, alumina, and other dopants using conventional processes, such as chemical vapor deposition, that are amenable to dopants, such as Ge or Ti, having relatively high vapor pressure precursors.
Yet another problem that occurs in vapor deposition of dopants that sublimate rather than vaporize is the likelihood of bubble formation. Oftentimes, when the dopant system includes solids rather than liquids, solid dopant particles can be carried to the reaction zone by carrier gases during the soot lay-down step. Since the dwell time of solid particles in the heat source or flame is minimal, the solid particles cannot be completely reacted or oxidized. These unreacted solid particles may attach to the glass particulate, or soot, and become incorporated into the soot blank. The particles eventually react and decompose in subsequent processing steps involving elevated temperatures. The decomposition of these solid particles may cause gas bubbles of Cl2, or other gaseous by-products in the resultant preform or preform core.
To alleviate some of these above noted problems, other dopant methods such as solution doping, may be used. These methods, although effective, are also not without drawbacks. In solution doping, such as the process disclosed in U.S. Pat. No. 3,859,073, the porous glass optical fiber preform is immersed within a solution containing the dopant for a period sufficient for the dopant to be incorporated into the blank. The preform is dried for a certain time and then consolidated to form a glass article. Porosity of the preform must be tightly controlled to ensure that the dopant solution can be absorbed into the preform. If the particles within the porous preform are too loosely bound, the preform can disintegrate or crack within the solution. Further, the preform must be thoroughly dried prior to consolidation to avoid cracking or other damage. Thus, the solution doping method requires extra processing steps, adds cycle time, and potentially introduces manufacturing defects into the fiber process.
The novel methods of the present invention control dopant profiles in glass articles comprising at least two dopants. More specifically, the present invention discloses novel methods for doping an optical fiber glass preform or glass article comprising a first dopant, such as GeO2, TiO2, P2O5, and/or B2O3, with a second dopant, such as an oxide of Al, Zr, Y, Nb, Ta, Ga, In, Sn, Sb, Bi, the 4f rare earths (atomic numbers 57-71 of the periodic table), Be, Mg, Ca, Zn, Sr, Cd, and Ba, so that the concentration of the second dopant is influenced by the concentration of the first dopant. As a result, the second dopant within the consolidated preform or glass exhibits a radial profile that correlates with, or substantially mirrors, that of the first dopant contained within the initial soot preformn. Further, the extent of radial penetration of the second dopant may be substantially equal to that of the first dopant. Moreover, the concentration of the added second dopant is higher than is expected without the presence of the first dopant.
The methods of present invention provide for the controlled incorporation of a doping chemical species not readily achieved during the initial fabrication of the glass. Accordingly, one aspect of the present invention is directed to methods for making an optical fiber glass preform having a radially uniform dopant profile and the optical fiber made therefrom. A further aspect of the present invention is directed to methods for controlling, or influencing, dopant profiles. Lastly, an additional aspect of the present invention is to effect the extent of radial penetration of the added dopant.
Yet another aspect of the present invention provides methods of improving the concentration of dopants within a glass article utilizing vapor infiltration doping techniques. Vapor infiltration processes provide a solution to incorporating dopants, such as the oxides of Al, Cd, Zn, Zr, Y, Nb, Ta, Ga, In, Sn, Sb, Bi, the 4f rare earths (atomic numbers 57-71 of the periodic table), and the alkaline earths Be, Mg, Ca, Sr, and Ba, into optical fiber glass preforms or cores, that cannot be incorporated, or incorporated with great difficulty, into the glass during the initial formation of the soot blank. The doping cycle occurs after a porous glass blank containing a first dopant, typically GeO2, TiO2, P2O5, or B2O3, is formed. The present invention provides methods for improving the dopant profiles, respective dopant concentrations, and degree of dopant penetration through careful tailoring of the amount of GeO2, TiO2, and/or other non-SiO2 oxides present in the porous glass blank.
The vapor infiltration techniques of the present invention comprise placing a previously formed porous glass blank containing a first dopant, such as GeO2, TiO2, or P2O5, within a furnace that can maintain the blank at a controlled isothermal temperature. A second dopant, such as a metal from the group consisting of Al, Cd, Zn, Zr, Y, Nb, Ta, Ga, In, Sn, Sb, Bi, the 4f rare earths (atomic numbers 57-71 of the periodic table), and the alkaline earths Be, Mg, Ca, Sr, and Ba, is reacted with a halide gas to form a metal vapor. The metal vapor comprising the second dopant infiltrates the porous glass blank containing the first dopant. During the vapor infiltration process, the second dopant preferentially reacts with the first dopant rather than the SiO2 present within the porous glass blank. The concentration and radial profile of the second dopant thereby follows the concentration and radial profile of the first dopant.
In some preferred embodiments, a gas impermeable doping tube is inserted into the central bore of the porous blank or porous cylinder. The doping tube has an open end and a closed end. The closed end of the tube, or dosing tip, may be comprised of a porous material or glass with one or more slots. The dosing tip also contains a reservoir for the metal dopant source. A gas, such as chlorine, flows through the dosing tip and over the metal dopant source thereby reacting with the dopant to form a dopant vapor. The dopant vapor flows through the pores or slots at the end of the tube and infiltrates the pores of the preform. The dosing tip is traversed up and down across the length of the blank. The dosing tip is also rotated about the blank""s longitudinal axis. After the dosing step is completed, the blank undergoes drying and sintering procedures to form a fully dense glass which includes the oxide form of the dopants. The glass can then be drawn directly into an optical fiber or coated with an additional glass layer or layers to achieve desired optical properties.
Several important advantages are achieved by making glass articles, such as planar waveguides, optical waveguide preforms, or waveguide core preforms (also referred to as canes or rods), using the methods of the invention. One advantage is an increase in the overall concentration of a second dopant into a glass blank containing a first dopant, such as GeO2 and/or TiO2. A further advantage of the present invention is to effect the penetration of the second dopant that extends radially, i.e. from the center of the preform, to a greater degree than is generally achieved with other known methods. Yet a further advantage of the present invention is providing a cane or preform that exhibits a substantially uniform dopant profile across its radius. These advantages enhance the performance of the optical fiber or planar waveguide for glass optical fiber amplifiers, lightwave optical circuits, or other applications.
It is to be understood that both the foregoing general description and the following detailed description are exemplary and are intended to provide further explanation of the invention as claimed. The accompanying drawings are included to provide a further understanding of the invention. In the drawings, like reference characters denote similar elements throughout several views. It is to be understood that various elements of the drawings are not intended to be drawn to scale.
A more complete understanding of the present invention, as well as further features and advantages of the invention such as its application to other methods of fabricating fiber optic waveguide or preforms and fibers or other refractive index profiles of fibers, will be apparent from the following Detailed Description and the accompanying drawings.